The present invention relates to an optical storage medium which is capable of performing optical read and write of information by using an organic dye.
As one system for optical recording using a thin film of an organic dye as recording medium, there has been proposed a recording system comprising an optical storage medium which makes use of J aggregation of a dye, as for example, Japanese Patent Application Kokai (Laid-Open) No. 205187/86.
J aggregation is specified by the fact that it shows an absorption spectrum with a sharp peak, namely a harrow half-width of peak wavelength in the aggregate state of dye, and helps to enhance the wavelength selectivity of recording layer.
The principle of the recording mode with the optical recording medium is such that the medium is exothermally heated by irradiating with a laser beam having an oscillation wavelength corresponding to the peak absorption of the medium to allow the medium to absorb the beam, whereby certain thermal modes are caused in the heated medium resulting in variation in structure of the medium which affords an extinction or diminution of corresponding peak signals. Thus, the produced variations in the absorbance can be stored as signals. Furthermore, for effecting the high-density recording of information by the optical recording technique using the above recording mode, a multi-frequency recording system is disclosed in U.S. Pat. No. 4,737,427, in which a multiple-bit information is recorded in a single recording site by making use of a plurality of dyes differing in maximum absorption wavelength.
An optical storage medium which makes use of the aggregation of dye molecules has high recording sensitivity and is capable of making recording with a small recording energy because of a large .gamma. value (".gamma. value" referred to herein designates, for example, the gradient of substantially straight-line section of the recording characteristic curve representing the change of absorbance in relation to applied energy as shown in FIG. 3) and a small recording threshold value. However, since this medium is so sensitive to even a slight variation in the recording energy due to the large .gamma. value that it undergoes a change to a large extent with a slight fluctuation of the recording laser beam, or in the optical system, many deviations from the normal rate of variation in the absorbance of the medium tend to be caused at the time of recording, which results in an instability in recording. There are still other problems at the time of reading out the recorded information, wherein the energies of the lights used tend to cause unintended recordings in the medium due to the small recording threshold value which readily results in a change in the storage state or disappearance thereof, that is, in a loss of recorded information.
In conventional optical storage media, the recording sensitivity characteristic determining the performance of the medium has been decided by the dye material itself and could not be controlled by the constitution of the storage medium.